Laser shock processed pilger dies

ABSTRACT

A pilger die construction having a laser shock processed area. The processed area includes at least one of the following pilger die locations: the taper, the flank, the sizing area, the tread, the groove. A method of laser shock processing a pilger die by controlling the application of a transparent overlay on the surface of the pilger die is also disclosed.

CROSS REFERENCE

The present utility patent application related to claims priority from Provisional U.S. Patent Application No. 60/758,338 filed Jan. 12, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the cold forming or pilgering of tubular members and particularly to the laser shock processing of the dies used in such forming process.

2. Description of the Related Art

In the fabrication of tubular cladding for use in fuel elements of nuclear reactors, such cladding is normally made by pilgering of zirconium alloy cylinders. The extruded tubular shape of such cylinders is reduced in cross-sectional dimensions, both in diameter and in wall thickness by multiple pilger reductions, generally at room temperature.

More Generally, pilgering is a conventional tub forming operation by which a tube is simultaneously advanced over a stationary mandrel and compressed using two opposing roller dies resulting in the reduction of the cross-sectional area and in elongation of the tube.

Typically, the input tube is reduced and elongated to the final tube by passing through a succession of stations of the pilgering machine with each station being composed of a stationary mandrel and roller die set. Reduction is effected in both the diameter and wall thickness of the tube by means of a tapered shape of the mandrel and the circumferential tapered shape of the grooves in the dies which embrace the tube from above and below the mandrel and roll in a constant cycle back and forth along the tube. Between each cycle of the die movement, the tube is advanced and rotated incrementally along the mandrel. The mandrel prevents the tube from collapsing under the force of the roller dies while at the same time dictates the inner diameter of the tube.

Although the mandrels and roller dies are fabricated from high strength steel, a limiting factor in the pilgering operation is the need for frequent replacement of mandrels and roller dies. Mandrel replacement is required when the steel mandrels become overstressed and break from severe operating conditions and occasional bending moments imposed thereon by tube eccentricity or slight misalignment. Roller die replacement is frequently required due to occurrence of surface cracks, fretting and spalling in the die grooves of the steel dies as a result of severe operating conditions of the pilgering machine.

A significant cost in this operation is the life span of the pilger dies. Previous indications show that such dies typically fail by surface spalling similar to that in bearings due to the high compressive stresses normal to the groove surface. Higher harness dies such as those formed by SR1855 tool grade steel with a hardness of about 58 Rockwell C, resists spalling failures. These higher stress dies fail, however, by cracking due to the cyclic tensile stresses produced in the surface of the die groove. These tensile stresses are produced by the resolution of the working stresses against the groove surface in the transverse direction with respect to the groove axis. Previous approaches have been identified to deal with the cyclic tensile stresses in the pilger dies.

Case hardening has been one method, along with directional quenching, heat treatment processes, along with other approaches dealing with the tensile operation stresses allowing for elastic deflection under load to produce compressive residual stresses in the groove area. Case hardening had a feature in that only a surface layer is hardened by the martensitic reaction in steel, while the bulk of the die remains in a soft, untransformed condition. Residual stresses are of consequence in thermal contraction on cooling of a volume expansion produced by the martensitic hardening reaction. That is, during the quenching operation, the rapid cooling of the surface results in the martensitic hardening reaction in this volume expansion as the interior of the die cools it can also transform to martensite (resulting therefrom in through hardening) in continue to cool and thermally contract. If the interior of the die also transforms to martensite, the accompanying volume expansion forces the already cooled and hardened surface to be displaced outwardly, resulting in residual tensile stresses in the surfaces of the die. If the interior does not transform, but continues to thermally contract, the accompanying contraction causes the residual compressive stresses in the surface of the core and case to increase. These two conditions have been shown to have a dramatic effect of pilger die life with a very inferior die life demonstration demonstrated in through hardened dies. Residual stress and thus die life in productivity is determined primarily by the change in volume of the interior or core during quenching.

What is needed in the art is a pilger die with an increase lifetime of utilization.

SUMMARY OF THE INVENTION

A die for a pilgering apparatus for use in reducing a tube comprises a ring shaped member that consists of hardened steel alloy which is hardened by transforming to martensite under predetermined heat treating conditions. In form of the invention, laser shock processing is applied to the operable working groove of the pilger dies to thereby increase the level of residual compressive stresses therein. In other forms of the invention, other locations of the pilger die are laser shock processed.

In one embodiment of the present invention, only the narrow throat section of the working group is laser shock processed. The pilger die itself may be composed of one, two, or more alloys, as is known in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a pilgering die of the present invention;

FIG. 2 is a plan view of the die of FIG. 1;

FIG. 3 is a view inside elevation of the die shown in FIG. 2;

FIG. 4 is a view in longitudinal section taken along line 4-4 of FIG. 2;

FIG. 5 is a view in longitudinal section taken along line 4-4 of FIG. 2, similar to FIG. 4, of an alternate embodiment; and

FIG. 6 is a diagrammatic view of the laser shock processing apparatus used in conjunction with the pilger die of FIG. 2.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

The present dies are distinct from previous pilger dies wherein residual stress control as previously described include involving a tool steel alloy of a single or multiple composition and only controlling the cooling rate within the tool during heat treatment.

FIG. 1 illustrates a pilger die 10 which comprises an annular roll or ring having a bore 13 therethrough for mounting on a shaft. The outer periphery P of the ring has a tapered groove 15, a circular configuration therealong. The bore 13 has a keyway 17 for engagement of a key in a shaft upon which the die is to be mounted to suppress the tendency of the die to rotate relative to the shaft when the die is subjected to high pressure in operation. The transverse cross-section of the groove 15 each position along the periphery of die 10 is a circular arc, approximating a semi-circular arc. Over at least a portion of the periphery of the radius of the arc varies from a magnitude slightly greater than the starting OD of the tube to be reduced to a smaller magnitude slightly less than the OD of the tube following reduction. The groove extends beyond the taper from the smaller radius end from an appreciable distance. This extension is called the sizing area. The groove also extends from the larger radius end of the taper. At this end, the radius of the groove is enlarged to prevent tube/tool contact and to facilitate the feeding rotation of the tube. Cylindrical surface of the die 10 extending from the groove is called the tread and the sides of the die are called the flanks. The shape of the groove 15 is illustrated in FIGS. 1-3. As illustrated in the cross-sectional view of FIG. 4, the pilger die of the present invention consists of at least one location of a laser shock processed area.

FIG. 4 discloses an embodiment of the invention utilizing a pilger die consisting of at least two steel alloys. A first alloy section 9 is composed of a hardened first steel alloy that is transformed to martensite under predetermined heat treating conditions. This section of hardened steel alloy extends from the outer periphery inwardly towards the bore 13 of the die beyond the predetermined distance d of the groove 5 to a point 23 spaced from the bore 3. The second alloy section 11 is composed of a second steel alloy that does not transform to martensite when exposed to the heat treating conditions needed to transform the first alloy to martensite. By utilizing a die which consists of two alloy sections, a control of the volume change and thus residual stress during heat treatment is effected. A second steel alloy is selected for section 11 or the core of the die which differs from the first steel alloy for section 9 or case of the die to obtain desired volume contraction during the heat treatment quench or cool down of the die which effect hardening of the case or first alloy section of the die. Thus, the case or first section 9 is still composed of a tool steel alloy with the desired strength or hardness to resist the high compressive operating stresses on the die during pilgering operations. The second alloy section or core, however, is composed of an alloy with the characteristics necessary to produce the desired volume contraction resulting in the desired residual stresses in case to resist the cyclic operating tensile stresses which occur during a pilgering operation. The second alloy selected for the second section 11 or core of the die can be a steel alloy similar to that of the first section 9 or case except that its chemical composition (i.e. low carbon content) would prevent the martensite hardening reaction and thus guarantee volume contraction in the second section or core on quenching and a residual compressive stress in the first section enabled by the laser shock processing system (to be discussed infra).

FIG. 5 shows use of a third alloy section 25 is provided at the bore 13 of the die, in addition to the first and second alloy sections. In this embodiment, a hardenable alloy which is hardenable to martensite under predetermined heat treating conditions is provided at the periphery P of the die and at the bore 13, with the second alloy section therebetween composed of a non-hardenable alloy comprised of material that does not transform to martensite when exposed to said predetermined heat treating conditions. As illustrated, the die 10 here in FIG. 5, comprises an annular ring having a bore 13 with a groove 15 in the outer periphery P thereof. A first alloy section 9 of a hardened steel alloy that is transformed to martensite under predetermined heat treating conditions extends from the outer periphery, beyond the depth d of the groove 5, to a point 23 spaced from the bore 13. A third alloy section 25 is also composed of a hardened steel alloy that is transformed to martensite under the predetermined heat treating conditions, and extends from the bore 13, a distance d′ to a point 27 spaced from the first alloy section. The second steel alloy section 21 is composed of the second steel alloy that does not transform to martensite when exposed to the heat treating conditions needed to transform the first alloy and the third alloy to martensite. Thus, a non-hardenable steel alloy having a hardness of between about 35 Rc to 45 Rc, or other preferred ranges, is sandwiched between a first alloy section or case and a third alloy section or bore, the latter two alloys having a hardness of 53 Rc to 63 Rc preferably although other ranges may be utilized.

More particularly, the present invention includes the provision of individually and separately laser shock processing at least one of the portions of the pilger die known as the taper, flank, groove, sizing area, and tread Further, the present invention includes laser shock processing any combination and permutation of the aforementioned locations on the pilger die. Examples of such combinations and permutations, without limitation include laser shock processing the taper and flank, sizing area and tread, taper and sizing area, or even three or more element combinations. In one form of the invention, the invention includes the method of creating a laser shock processed pilger die.

The laser shock processing system utilized in the present invention is shown in FIG. 6.

For a more through background in the prior history of laser peening and that of high power processing of engineered materials, reference can be made to U.S. Pat. No. 5,131,957, such patent explicitly hereby incorporated by reference. This patent also shows a type of laser and laser circuit adaptable for use with the present invention. Another type of laser adaptable for use with the invention is that of a Nd:Glass Laser manufactured by LSP Technologies of Dublin, Ohio.

Overlays are applied to the surface of the pilger die workpiece being laser peened. These overlay materials may be of two types, one transparent to laser radiation and the other opaque to laser radiation. They may be used either alone or in combination with each other, but it is preferred that they be used in combination with the opaque overlay adjacent the pilger die workpiece, and the outer, transparent overlay being adjacent the opaque overlay.

The transparent overlay material should be substantially transparent to the radiation. Useful transparent overlay materials include water, water-based solutions, other noncorrosive liquids, glass, quartz, sodium silicate, fused silica, potassium chloride, sodium chloride, polyethylene, fluoroplastics, nitrocellulose, and mixtures thereof. Fluoroplastics, as they are known by ASTM nomenclature, are parallinic hydrocarbon polymers in which all or part of each hydrogen atom has been replaced with a fluorine atom. Another halogen, chlorine, can also be part of the structure of a fluoroplastic. By order of decreasing fluorine substitution and increasing processability, these materials include polytetrafluoroethylene (PTFE); fluorinated ethylenepropylene (FEP): the chlorotrifluorethylenes (CTFE); and polyvinylidine fluoride (PVF.sub.2). Also available is a variety of copolymers of both halogenated and fluorinated hydrocarbons, including fluorinated elastomers. Additionally, the transparent overlay could be a gel or a strip of tape comprised of one or more of the above materials. In the preferred embodiment of the present invention, water is used as the transparent overlay to confine the plasma created.

Where used, the opaque overlay material should be substantially opaque to the laser radiation. Useful opaque overlay materials include black paint, pentacrythritol tetranitrate (PETN); bismuth, lead, cadmium, tin, zinc, aluminum, graphite; and mixtures of charcoal or carbon black with various transparent materials such as mixtures of nitrocellulose and potassium perchlorate or potassium nitrate. Optionally, a layer of another solid overlay material may be attached to the layer of substantially opaque material. The outer, solid layer may be either transparent or opaque. The term “transparent” in this application is defined as meaning pervious to the laser beam utilized, not automatically or necessarily pervious to visible light. A typical overlay is between 10 micrometers and 20,000 micrometers (m) thick. In the preferred embodiment of the invention, water based black paint is used to give superior results both in terms of energy absorption and removability after laser peening.

Referring now to the drawings and particularly to FIG. 6, there is shown a preferred embodiment 10 of the present invention including a laser-peening chamber 12 in which the laser peening takes place. The laser-peening chamber 12 includes an opening 14 for a laser beam 16 created by laser 18, a source of coherent energy. Laser 18, by way of example, may be a commercially available high power pulse laser system capable of delivering more than approximately 10 joules in 5 to 100 nanoseconds. The laser beam energy, pulse length and spot size on the pilger die workpiece may be adjusted. Of particular concern is sufficiently laser shock processing the pilger die in a particular location as discussed above. Another example area utilizing in testing, is that of an area of the groove not more than 1 ⅜ inch of groove length, starting from the deep groove, or tread, and more than one half the groove width. The tests were conducted for the most part on 5.6″ diameter pilger dies, in which the groove depth and radius were approximately 3/16″.

Shown in FIG. 6, a workpiece 10 is held in position within laser-peening chamber 12 by means of a positioning mechanism 22. Positioning mechanism 22 may be of the type of a robotically controlled arm or other apparatus to precisely position workpiece 10 relative to the operational elements of laser shock system 30.

Laser peening system 30 may include a material applicator 24 for applying an energy absorbing material onto workpiece 10 to create a coated portion. Material applicator 24 may be that of a solenoid operated painting station or other construction such as a jet spray or aerosol unit to provide a small, coated area onto workpiece 20. The material utilized by material applicator 24 is an energy absorbing material, preferably that of a black, water-based paint such as 1000 F AQUATEMP ™ from Zynolite Product Company of Carson, Calif. Another opaque overlay that may be utilized is that of ANTI-BOND, a water soluble gum solution, including graphite and glycerol from Metco Company, a Division of Perkin-Elmer of Westbury, N.Y. Alternatively, other types of opaque overlays may be used such as those discussed above.

Laser peening system 30 further includes a transparent overlay applicator 26 that applies a fluid or liquid transparent overlay to workpiece 10 over the portion coated by opaque overlay applicator 24. The transparent overlay material should be substantially transparent to the radiation as discussed above, water being the preferred overlay material.

A control unit, such as controller 28 is operatively associated with each of the opaque overlay material applicator 24, transparent overlay material applicator 26, measurement device 5, tamping device 45, laser 18 and positioning mechanism 22. Controller 28 controls the operation and timing of each of the applicators 24, 26, tamping device 45, laser 18 and selective operation of positioning mechanism 22 to ensure proper sequence and timing of laser peening system 30. In addition, controller 28 acquires the thickness and uniformity measurements from measurement device 5. Shown in FIG. 6, controller 28 is connected to laser 18, positioning mechanism 22, opaque overlay material applicator 24, transparent overlay material applicator 26, measurement device 5 and tamping device 45 via control lines 40, 32, 34,36, 17 and 27, respectively. Controller 28 is also connected to control valves 29 and 35 via lines 31 and 33 respectively, and to control material input lines 19 and 21 for applicators 24 and 26, respectively. Controller 28, in one embodiment, may be a programmed personal computer or microprocessor.

In operation, controller 28 controls operation of laser peening system 10 once initiated. The method of the invention is that first, workpiece 10 is located particularly within laser-peening chamber 12 by positioning mechanism 22. Controller 28 activates material applicator 24 to apply an energy absorbing overlay such as a water-based black paint onto a particular location of workpiece, in this particular case a pilger die 10 to be laser peened. Next the controller acquires a thickness and/or uniformity measurement of the paint from measurement device 45 and stores the value in controller 28. The next step of the process is that controller 28 causes transparent overlay material applicator 26 to apply transparent overlay to the previously coated portion of pilger die 10. The controller acquires a thickness measurement of the transparent overlay and stores this value in controller 28. At this point, controller 28 compares the measured thickness and uniformity values of the paint and overlay water to the specified values for each. These specified values may be a predetermined range of values based upon processing conditions or a specified range, which may be based upon a statistically determined range. If the measured values for the two overlays are within a specified range, laser 18 is immediately fired by controller 28 to initiate a laser beam 16 to impact the coated portion If the measured values are not within the specified range the controller 28 initiates a wash sequence and removes the overlays. The controller then makes adjustments to the applicator head valves 29 and 35 to achieve the desired thickness and uniformity values.

Additional features of the preferred laser peening system is shown in U.S. Pat. No. 6,841,755 assigned to the assignee of the present invention, which is explicitly incorporated herein by reference.

In a test of the laser peened pilger dies, the laser shock processed pilger dies lasted approximately twice times as long prior art dies until having observable cracking resulting in failure of the die and unsuitability for additional tube processing.

While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims. 

1. A pilger die having a laser shock processed area.
 2. The pilger die of claim 1, in which said processed area includes at least one of the following pilger die locations: the taper, the flank, the sizing area, the tread, the groove.
 3. The pilger die of claim 2 in which said processed area group is minimized to that only those needed for effective pilger lifetime extension.
 3. A method of laser shock processing a pilger die by controlling the application of a transparent overlay on the surface of the pilger die to reduce the variability of shock waves generated therein, comprising: applying a transparent overlay material over said surface of said pilger die; measuring the thickness of said transparent overlay in at least one location on said transparent overlay; determining if said measured value for said overlay is within the specified range; and directing a pulse of coherent energy to said pilger die to create a shock wave therein when said measured value is within the specified range. 